The present invention relates to the field of plasma processing of semiconductor wafers and more particularly to plasma processing equipment having improved and tunable chamber vacuum characteristics.
As is known in the art, a fundamental step in the manufacture of semiconductor devices, such as integrated circuits (ICs), is the process of forming electrical interconnections. The formation of electrical circuits, such as semiconductor transistors, involves a series of steps starting with the formation of a blank silicon wafer. The blank silicon wafer is then processed using successive steps of depositing to and etching away various materials to form the proper interconnections and therefore the electrical circuits.
One method of depositing and etching metal layers to and from a silicon wafer includes the use of a so-called plasma reactor system. In semiconductor manufacturing, plasma reactor systems are used to remove material from or deposit material to a workpiece (e.g., semiconductor wafer) in the process of making integrated circuit (IC) devices. A key factor in obtaining the highest yield and overall quality of ICs is the uniformity of the etching and deposition processes.
In a narrow gap, high aspect ratio capacitively coupled plasma reactor, a chuck design is often employed that attempts to allow the chuck (i.e. wafer work piece holder) to serve additional purposes other than supporting the wafer. The complexity of the mechanical design of the chuck is such that a non-optimal vacuum system design is required. A vacuum pumping system is used to evacuate the reactor processing region to the low pressures necessary to create a clean environment, to which a specific gas chemistry is introduced, which provides an environment for the generation of plasma. Consequently, due to the complexity of the chuck mechanical design, the symmetry of the vacuum system (relative to the wafer) is sacrificed such that the vacuum pump is usually positioned to access the reactor vacuum chamber from the side rather than from the chamber bottom or top.
This type of multi-purpose chuck can become a very cumbersome component of the reactor. In a multi-purpose chuck design, in addition to supporting the wafer, the chuck is typically configured to provide vertical translation in order to reduce the electrode-to-wafer spacing. This spacing control is necessary in order to produce a narrow gap for process conditions and to enlarge the gap for wafer exchange. In addition to the aforementioned capabilities, the chuck must be capable of sustaining a radio frequency (RF) energy bias. Moreover, the chuck design further includes components for chuck cooling, electrostatic clamping and backside gas flow to improve thermal conduction (between the wafer and the chuck). Consequently, the vacuum design is often a secondary consideration to other various mechanical and electrical component designs.
A reactor chamber that is equipped with a side mount vacuum port is considered an “asymmetrical design” in a nominally cylindrical system. An inherent drawback associated with an asymmetric design is that it often times produces an asymmetric process. One such asymmetry stemming from an asymmetric vacuum design is the observation of pressure field non-uniformity above the wafer when the chamber is evacuated from the side. That is, a pressure gradient with about 10-20% variation can occur across the wafer being processed. In general, for moderate to high pressures (e.g. P>20 mTorr), a region of low pressure is observed at an azimuthal location adjacent the pump entrance or pumping duct entrance (the pumping duct interfaces the inlet of the pump, e.g. turbo-molecular pump, with the vacuum chamber). In prior art capacitively coupled plasma reactors, attempts to solve the problem of an asymmetric chamber flow field introduced by pumping from the side have included the insertion of an orifice plate adjacent to the chuck.
A processing chamber generally includes a single evacuated volume wherein a portion of that volume is proximate the wafer and is hereinafter referred to as the processing region. When an orifice plate is employed, the chamber volume is separated into two regions by the orifice plate. The first region is predominantly occupied by the wafer processing region and the second region, referred to as the pumping volume, is accessed by the vacuum pump. This solution tends to improve the flowfield uniformity in the upper chamber volume by providing sufficient flow resistance through the orifice plate. However, as will be discussed later, this improvement is achieved at the expense of flow conductance or pumping speed at the processing region. In addition to placing the orifice plate adjacent the chuck, other prior art designs included locating the orifice plate adjacent other surfaces, e.g. any surface interfacing the processing chamber volume that allows the exhaust of chamber gases. For example, U.S. Pat. No. 4,209,357 to Gorin et al. discloses injection orifices that are integrated with the exhaust orifices upon the upper electrode. The “small” exhaust orifices serve the same purpose for providing flow resistance (or reduced flow conductance). Other prior art chambers include a design consisting of a narrow annular orifice surrounding the chuck or wafer where the narrow annular gap separates the processing region from the pumping region (see U.S. Pat. No. 4,352,974 to Mizutani et al., U.S. Pat. No. 4,367,114 to Steinberg et al., U.S. Pat. No. 5,441,568 to Cho et al., and U.S. Pat. No. 5,856,240 to Sinha et al.
There are several inherent problems with the methods employed in the prior art to use an orifice to control pressure uniformity in a chamber. For example, prior art orifice plates typically distribute the small openings equally in the azimuthal direction about the orifice plate in the hope that the resultant flow conductance will be azimuthally symmetric through the plate. However, in order to achieve flowfield uniformity, it is necessary to restrict the flow through the orifice plate to the extent that the pressure difference across the orifice is significantly greater than any pressure gradient in the processing or pumping regions. This requires making the holes in the orifice plate small and, hence, paying a penalty in chamber pumping speed at the wafer. This penalty in pumping speed directly results in an adverse effect on throughput. Additionally, the methods of flow restriction in the prior art are not adjustable, that is they are fixed in space for one set of conditions. The fact that they are fixed requires the system to be modeled for a particular orifice plate or tested, one orifice plate after another, to determine the optimal geometry.
In addition to the problem of pressure field non-uniformity described above, an additional problem associated with plasma processing systems is the transport of plasma to the pumping duct and pump inlet. In general, the aforementioned orifice plate or a separate pumping duct screen is utilized to attenuate the plasma density prior to reaching the pump inlet. For example, in typical prior art systems a pump screen (with generally less than 50% solidity) is placed in the cross-section of the pumping duct. Unfortunately the pumping screen attenuates the plasma and also reduces the pumping speed delivered to the processing region by at least a factor of two. This approach results in at least 50% of the frontal area of the pumping duct cross-section being utilized for recombination surfaces. In conventional designs, there is a one-to-one relationship between the increase in recombination surface area and the decrease in the frontal (flow-through) area.
It would be advantageous therefore to provide a reactor with a way to dynamically vary the flowfield in different segments of the reactor chamber.